In order to make an object having a foamed core and a dense skin which may be of a different resin from the core, it is necessary to use a nozzle having a pair of ports. The port from which the skin-forming denser resin is extruded is annular and surrounds a central port from which the core-forming resin having the blowing agent is extruded. Thus the foaming resin is extruded inside a sheath of the skin-forming resin.
The standard such machine, as described in German patent publication Nos. 2,259,818; 2,342,789; 2,342,794; 2,346,135; and 2,449,758 (U.S. equivalent patent 4,078,875), has a central nozzle part that can move axially relative to an outer nozzle part. The central nozzle part is formed at the axis with the central port for the core resin and the outer nozzle part defines with the central nozzle part an annular gap that constitutes the annular nozzle port from which the skin resin is extruded. The size of the annular nozzle port can be controlled, and in fact the annular nozzle can be completely closed by axially displacing the central nozzle part relative to the outer nozzle part. A standard blocking needle is in turn axially displaceable in the central nozzle part to open and close this port.
Such an arrangement therefore requires the use of separate actuators for opening and controlling the two nozzle ports. In addition the structure forming this nozzle must be formed with a complex arrangement of flow passages for the various resins. It takes a great deal of machining to produce these passages so that such nozzles are quite expensive.